For an MRI apparatus, the Cartesian sampling method for taking samples along trajectories parallel to the orthogonal coordinate axis of the k-space is often used as a scheme for acquiring echo signals on the k-space needed for image reconstruction. However, a method referred to as non-Cartesian sampling for taking samples along trajectories that are not parallel to the orthogonal coordinate axis of the k-space has been receiving attention in recent years, and one example is the PROPELLER sampling method (reference: Yasushi Watabe et al, “Atarasii Ragiaru Sucan: PROPELLER-hou”, INNERVISION (17•9) 29-31, 2002)
According to the PROPELLER sampling method, a unit region (hereinafter, referred to as the blade) comprising plural parallel trajectories having the origin of the k-space in between is given as one unit, and echo signals corresponding to almost the entire region of the k-space are measured by rotating blades about the origin of the k-space. Within each blade, plural echo signals are acquired by applying a phase encoding pulse as with normal measurements by the Cartesian sampling method.
Also, the parallel imaging method is used as a technique for shortening an imaging time needed for the MRI apparatus. According to the parallel imaging method, a multiple receiver coil formed by combining plural high-frequency receiver coil units (hereinafter, referred to as RF receiver coils) having local, inhomogeneous sensitivity distributions that differ from coil to coil is used, and echo signals at each RF receiver coil are measured by skipping phase encoding steps. Then, aliasing artifacts occurring on respective reconstructed images are removed using sensitivity distribution data of each RF receiver coil. This can reduce the number of echoes needed to reconstruct a single image, which can in turn shorten an imaging time by reducing the repetition number of times of the pulse sequence.
However, when the parallel imaging method is applied to the PROPELLER sampling method as above, it is necessary to remove aliasing artifacts by applying the generalized aliasing artifact development algorithm to the data of the entire k-space that has been measured by skipping, and this need extends a computation time considerably.
The reference cited above is silent about the application of the parallel imaging method to the PROPELLER sampling method and the problems arising in such a case as discussed above.